1. Field of the Invention
This invention relates to low-noise, high-speed output drivers and, more particularly, to a push-pull driver for the transmission of binary signals on a transmission line.
2. Background Art
Basic operation of high-speed data processing systems requires high-speed logic circuitry to perform complex logic operations in the shortest possible time. Additionally, data in the form of electrical pulses must be transmitted between circuits, such as memory devices and logic circuits, at a high rate of speed over conductors. Because of the high data rates, the conductors exhibit characteristics of transmission lines even though the conductors may be only inches in length, as, for example, between integrated circuit chips of a multi-chip module.
A characteristic of a transmission line is that a relatively large amount of current is required to effect a rapid change from a high voltage to a low voltage state and vice versa. The rapid change is particularly important where digital signals represented by voltage level changes are transmitted between circuits. MOS logic circuits are commonly used in present day high-speed computers and large capacity complimentary MOS transistors are used to supply the current to charge and discharge transmission lines. The devices are typically connected in series between terminals of a power supply, with the output of the driver taken from the node at which the devices are interconnected. That arrangement is often referred to as a push-pull driver. A problem with a circuit of that type is that the large current capacity transistors connected to a transmission line require a significant period of time to turn off and on. For high-speed operations, it's desirable to switch the transistors as quickly as possible. Preferably, they are switched simultaneously, i.e. by turning on one transistor while turning off the other transistor. However, this requires a large current supply to drive. Furthermore, the simultaneous operation of the two switches also results in current spikes, referred to in "totem pole" currents, which cause so-called Delta-I noise. The Delta-I noise may cause extraneous signals which result in computation errors, degrading system performance.
Various attempts have been made to reduce the current spikes which produce the undesirable Delta-I noise. For example, U.S. Pat. No. 4,719,369 describes a CMOS output circuit which is adapted to adjust the driver's output impedance to match that of the transmission line. This arrangement is intended to minimize the variations of output swing and, hence, the current pulses. However, this arrangement requires separate width control circuitry and cannot be effectively used to eliminate totem-pole current if a fast driver design is desired. U.S. Pat. No. 4,961,010 describes an arrangement for reducing the undesirable noise spikes in a push-pull driver by partitioning of the output devices to allow the switches to be operated in sequence rather than simultaneously. However, this defeats the purpose of the high-speed circuit design, in that a delay is intentionally introduced. That circuit does not allow for a fast driver operating at speeds compatible with high-speed logic circuitry.